To date, all wet flue gas desulfurization systems having a tray or grid therein also incorporate an underspray header or other quencher, presaturator, or gas cooling device that is used to humidify the gas and to prevent the build-up of solids upon the underside of the tray or grid. In the past, it had been believed that the incoming flue gas needed to be sprayed via such an underspray header assembly in order to improve the operation of the FGD scrubber. To accommodate this belief, separate quench sections were added to the scrubber tower. However, after experimentation, it has now come to light that an underspray header in towers incorporating a tray or grid is no longer needed and that a wet scrubber tower without an underspray header operates equivalent to a tower which includes such a device.
Generally, in a wet FGD tower, the bottoms product or reagent slurry is recirculated between the under and over spray headers. Usually, about 10%-40% of the slurry is delivered to the underspray header for the purpose of saturating and humidifying the incoming gas while the remaining 60%-90% or so is delivered to the overspray header for the purpose of pollutant removal.
As previously indicated, the use of an underspray header was thought necessary so as to uniformly humidify the incoming flue gas and so that the flue gas reaching the tray or grid had a known moisture content. This control over the physical and chemical characteristics of the incoming flue gas was believed essential for proper operation of the scrubber tower and for proper chemical reaction above the tray or grid. Also, the underspray header was often used to continuously wash the bottom of the tray or grid so as to prevent any build-up from occurring at what is known as the wet/dry interface. This wet/dry interface is a zone in the system where the incoming dry flue gas mixes with the humidifying liquid. Unfortunately, the inclusion of an underspray header increases the height of the scrubber tower by as much as two feet or more. Such a scrubber tower requires more height and volume thereby consuming more space in a facility in which excess space adds significantly to the cost. Also, such an underspray header assembly increases the cost of constructing the tower since more piping, pumps, valves, nozzles, and the like are required. Additionally, an underspray header increases the cost of operating the tower due to a greater demand for power and energy. Furthermore, underspray headers increase the final elevation of the overspray headers thereby resulting in a proportionate increase in pump power necessary to supply the bottoms product or reagent slurry to the overspray headers.
By eliminating the underspray header, all of the bottoms product is now directed to the overspray header with the rain falling from the tray or grid under the overspray header being used to humidify and saturate the incoming flue gas, a step normally accomplished by the underspray header. Also, the tray evenly distributes the weeping rain across the tower thereby maintaining uniform gas distribution within the tower. Furthermore, the portion of rain from the tray falling upon an awning structure just above the flue gas inlet forms a continuous, generally uniform curtain of liquid across this inlet. This curtain provides the primary means of gas quenching while the awning protects the floor and walls of the flue gas inlet from any backflow of slurry into the inlet.
It is thus an object of the present invention to provide more reagent on top of the tray or grid where the contaminant removal efficiency is greater and more effective. Another object of the present invention is to provide a shorter tower that is less costly to build, furnish, and operate. Yet another object of the invention is to humidify and saturate the incoming flue gas by directing it through a continuous curtain of reagent falling from an awning shielding the inlet flue gas. Another object of the invention is to direct the gas through a continuous flow of rain from the tray or grid. Still another object of the present invention is to prevent any sideways swirl that may arise upon the entrance of the flue gas into the tower. Furthermore, an object of the present invention is to provide a scrubber tower having high removal efficiencies equivalent to those towers which incorporate an underspray header assembly with less liquid recirculation. Still another object of the invention is to prevent build-up from occurring upon the wet/dry interface at the bottom of the tray or grid. These and other objects and advantages of this invention will become obvious upon further investigation.